R Custom proxy function in dtw for wraping - r

Question

I have two column of data Tm and Ts and I want to apply the dtw algorithm changing the distance function. Proxy provide this possibility but I can't understand why it gives me an error.
I have 2 vector of data with the same length:
Tm Ts
301.0607 300.6008
301.3406 300.6515
301.5912 300.7289
301.5777 300.8506
301.5996 301.0158
301.6414 301.2103
301.7181 301.4113
myDTW<-function(x,y)(diff(x,lag=1,difference=1)-diff(y,lag=1,difference=1))^2
pr_DB$set_entry(FUN = myDTW, names = c("test_myDTW", "myDTW"))
Alignment<-dtw(a$Ts,b$Tm,dist.method="test_myDTW",keep.internals=TRUE)
Error in do.call(".External", c(list(CFUN, x, y, pairwise,
if (!is.function(method)) get(method) else method), :
not a scalar return value
diff() changes the length of the vector from n to n-1 but both vectors are changed, so I think that the problem are not on matching vector of different length.
Do you have any suggestion?

Answer 1

The error is explicit :
not a scalar return value
Your myDTW don't return a scalar. You need to define it as a valid distance function. If you change it to something like :
myDTW <- function(x,y){
res <- (diff(x,lag=1,difference=1)
-diff(y,lag=1,difference=1))^2
sum(res) ## I return the sum of square here
}
It will works. I think also you need to use modify_entry to modify the method value in the register.
dat <- read.table(text='Tm Ts
301.0607 300.6008
301.3406 300.6515
301.5912 300.7289
301.5777 300.8506
301.5996 301.0158
301.6414 301.2103
301.7181 301.4113',header=TRUE)
myDTW <- function(x,y){
res <- (diff(x,lag=1,difference=1)
-diff(y,lag=1,difference=1))^2
sum(res)
}
pr_DB$modify_entry(FUN = myDTW, names = c("test_myDTW", "myDTW"))
library(dtw)
## I change a and b to dat here
dtw(dat$Ts,dat$Tm,dist.method="test_myDTW",keep.internals=TRUE)
The result is :
DTW alignment object
Alignment size (query x reference): 7 x 7
Call: dtw(x = dat$Ts, y = dat$Tm, dist.method = "test_myDTW", keep.internals = TRUE)